[+cc coly] On Mon, 5 Oct 2020, Eric Wheeler wrote: > On Sun, 4 Oct 2020, Kai Krakow wrote: > > > Hey Nix! > > > > Apparently, `git send-email` probably swallowed the patch 0/3 message for you. > > > > It was about adding one additional patch which reduced boot time for > > me with idle mode active by a factor of 2. > > > > You can look at it here: > > https://github.com/kakra/linux/pull/4 > > > > It's "bcache: Only skip data request in io_prio bypass mode" just if > > you're curious. > > > > Regards, > > Kai > > > > Am So., 4. Okt. 2020 um 15:19 Uhr schrieb Nix <nix@xxxxxxxxxxxxx>: > > > > > > On 3 Oct 2020, Kai Krakow spake thusly: > > > > > > > Having idle IOs bypass the cache can increase performance elsewhere > > > > since you probably don't care about their performance. In addition, > > > > this prevents idle IOs from promoting into (polluting) your cache and > > > > evicting blocks that are more important elsewhere. > > > > > > FYI, stats from 20 days of uptime with this patch live in a stack with > > > XFS above it and md/RAID-6 below (20 days being the time since the last > > > reboot: I've been running this patch for years with older kernels > > > without incident): > > > > > > stats_total/bypassed: 282.2G > > > stats_total/cache_bypass_hits: 123808 > > > stats_total/cache_bypass_misses: 400813 > > > stats_total/cache_hit_ratio: 53 > > > stats_total/cache_hits: 9284282 > > > stats_total/cache_miss_collisions: 51582 > > > stats_total/cache_misses: 8183822 > > > stats_total/cache_readaheads: 0 > > > written: 168.6G > > > > > > ... so it's still saving a lot of seeking. This is despite having > > > backups running every three hours (in idle mode), and the usual updatedb > > > runs, etc, plus, well, actual work which sometimes involves huge greps > > > etc: I also tend to do big cp -al's of transient stuff like build dirs > > > in idle mode to suppress caching, because the build dir will be deleted > > > long before it expires from the page cache. > > > > > > The SSD, which is an Intel DC S3510 and is thus read-biased rather than > > > write-biased (not ideal for this use-case: whoops, I misread the > > > datasheet), says > > > > > > EnduranceAnalyzer : 506.90 years > > > > > > despite also housing all the XFS journals. I am... not worried about the > > > SSD wearing out. It'll outlast everything else at this rate. It'll > > > probably outlast the machine's case and the floor the machine sits on. > > > It'll certainly outlast me (or at least last long enough to be discarded > > > by reason of being totally obsolete). Given that I really really don't > > > want to ever have to replace it (and no doubt screw up replacing it and > > > wreck the machine), this is excellent. > > > > > > (When I had to run without the ioprio patch, the expected SSD lifetime > > > and cache hit rate both plunged. It was still years, but enough years > > > that it could potentially have worn out before the rest of the machine > > > did. Using ioprio for this might be a bit of an abuse of ioprio, and > > > really some other mechanism might be better, but in the absence of such > > > a mechanism, ioprio *is*, at least for me, fairly tightly correlated > > > with whether I'm going to want to wait for I/O from the same block in > > > future.) > > > From Nix on 10/03 at 5:39 AM PST > > I suppose. I'm not sure we don't want to skip even that for truly > > idle-time I/Os, though: booting is one thing, but do you want all the > > metadata associated with random deep directory trees you access once a > > year to be stored in your SSD's limited space, pushing out data you > > might actually use, because the idle-time backup traversed those trees? > > I know I don't. The whole point of idle-time I/O is that you don't care > > how fast it returns. If backing it up is speeding things up, I'd be > > interested in knowing why... what this is really saying is that metadata > > should be considered important even if the user says it isn't! > > > > (I guess this is helping because of metadata that is read by idle I/Os > > first, but then non-idle ones later, in which case for anyone who runs > > backups this is just priming the cache with all metadata on the disk. > > Why not just run a non-idle-time cronjob to do that in the middle of the > > night if it's beneficial?) > > (It did not look like this was being CC'd to the list so I have pasted the > relevant bits of conversation. Kai, please resend your patch set and CC > the list linux-bcache@xxxxxxxxxxxxxxx) > > I am glad that people are still making effective use of this patch! > > It works great unless you are using mq-scsi (or perhaps mq-dm). For the > multi-queue systems out there, ioprio does not seem to pass down through > the stack into bcache, probably because it is passed through a worker > thread for the submission or some other detail that I have not researched. > > Long ago others had concerns using ioprio as the mechanism for cache > hinting, so what does everyone think about implementing cgroup inside of > bcache? From what I can tell, cgroups have a stronger binding to an IO > than ioprio hints. > > I think there are several per-cgroup tunables that could be useful. Here > are the ones that I can think of, please chime in if anyone can think of > others: > - should_bypass_write > - should_bypass_read > - should_bypass_meta > - should_bypass_read_ahead > - should_writeback > - should_writeback_meta > - should_cache_read > - sequential_cutoff > > Indeed, some of these could be combined into a single multi-valued cgroup > option such as: > - should_bypass = read,write,meta Hi Coly, Do you have any comments on the best cgroup implementation for bcache? What other per-process cgroup parameters might be useful for tuning bcache behavior to various workloads? -Eric -- Eric Wheeler
